1. Field of the Invention
This invention relates to new thermally crosslinkable one-component coating compositions containing blocked isocyanate groups and to their use for the production of highly elastic polyurethane coatings.
2. Description of the Prior Art
The use of blocked isocyanates for the production of thermosetting polyurethane coatings is known (cf. for example Kunststoff-Handbuch, Vol. VII, Polyurethane, Carl Hanser Verlag Munich, 1966, pages 11-13 and 21 et seq.). Numerous compounds capable of reversibly blocking isocyanates have been described. Among these compounds, particular importance is attributed to the class of C-H-acid compounds (examples: malonic acid and acetoacetic acid esters) by virtue of their favorable chemical and physiological properties (cf. for example German Offenlegungsschrift Nos. 2,342,603; 2,436,872; 2,550,156; 2,612,783; 2,612,784 and 2,612,785). German Offenlegungsschrift No. 2,550,156 and German Auslegeschrift Nos. 2,623,081 and 2,639,491 describe polyurethane stoving lacquers based on aliphatic polyisocyanates blocked by malonic acid esters or acetoacetic acid esters and organic polyhydroxyl compounds.
The systems described in the cited publications have the typical properties of surface lacquers, such as extreme hardness, gloss, resistance to yellowing and weathering.
However, previous tests (cf. the Comparison Tests described hereinafter) have shown that the polyurethane stoving lacquers according to the cited publications are not really suitable for the production of coatings characterized by very high shock and impact strength. However, there is an increasing need for coating compositions with which it is possible to produce shock- and impact-resistant coatings for protecting surfaces of industrial goods, such as for example machine parts, motor vehicle bodies or transport containers.
Hitherto, coating compositions based on bitumen or PVC or special amine cross-linked polyurethane systems (German Auslegeschrift No. 2,131,299) have been used for this purpose. The first of these systems based on bitumen or PVC are attended by the disadvantage that adequate protection is only guaranteed by substantial layer thicknesses of more than 1 mm. The result of this is that the coatings are very heavy and, because of their surface texture, cannot be high-gloss-lacquered. For reasons of appearance, therefore, they can only be applied to less visible parts of bodywork and not to every part requiring optimal protection against chipping by stones. Although the amine cross-linked polyurethane systems mentioned above guarantee a highly elastic protective surface coating in considerably thinner layers, they are attended by the disadvantage that they have to be stoved at relatively high energy consumption levels (60 minutes/140.degree.-160.degree. C. or 30 minutes/160.degree. C. or 10 minutes/180.degree. C.) and that low molecular weight amines can escape during the stoving process, releasing strong and, in addition, possibly toxic fumes.
The object of the present invention is to provide amine-free, toxicologically acceptable coating compositions which harden at comparatively low temperatures and guarantee highly elastic protective surface coatings in comparatively thin layers. In addition, the coating compositions are intended to be able to be applied as one-component compositions and to be liquid and stable in storage at room temperature despite their low solvent content.
According to the invention, this object is achieved by the provision of the coating compositions according to the invention described in detail hereinafter which contain as their polyisocyanate component certain dialkyl-malonate-blocked polyisocyanates of the diphenyl methane series and/or dialkyl-malonate-blocked NCO-prepolymers based on polyisocyanates of this type and, as their polyhydroxyl component, certain polyester polyols.
Dialkyl-malonate-blocked polyisocyanates of the type mentioned have hitherto been known as binders for moisture-hardening sealing compounds (U.S. Pat. No.3,779,794). On account of the totally different hardening mechanism of the systems described in this publication, they differ fundamentally from the coating compositions according to the invention described in detail hereinafter. In addition, the materials obtained in accordance with U.S. Pat. No. 3,779,794 are comparatively soft and mechanically weak with tensile strengths of less than 1 MPa and, although suitable for use as sealing compounds for fitting panes of glass, particularly in the car making industry, they cannot be used as coatings capable of withstanding severe mechanical stressing. By contrast, it is possible by the process according to the invention as described in detail hereinafter to produce coatings which have a tensile strength of the order of 20 to 30 MPa and, hence, are capable of withstanding severe mechanical stressing.